Starting and stopping your urine stream when you urinate will cause the muscles in the vagina walls to tighten up. This is a very good exercise to do. I believe it is called the kagel exercise or something like that.

Kegel exercises strengthen pelvic floor muscles, the muscles that stretch during childbirth. They help improve sexual pleasure (more intense orgasms) by making the muscles stronger and more elastic, and they can improve bladder tone so that you don't leak urine. They won't, however, actually make your vagina smaller, but they can make the opening tighter.

Here's how to do Kegel exercises:

When you're peeing, clench (without your hands) your muscles to stop the flow of urine for about four seconds. Then release those same muscles to let the urine flow again. These are your pelvic floor muscles; these are the muscles you'll be exercising when you do Kegels.
If you want to check and make sure that you're using the right muscles, put a finger or two into your vagina. Tighten the muscles. If you can feel your fingers being squeezed (even just a little), then you've located the right muscles.
About five to ten times a day, repeat the Kegels ten times per session.
It can take up to 8 – 10 weeks to notice improvement, and there will most certainly be improvement. Like any kind of weight lifting or muscle building exercises, Kegels take time to make a difference! (Adding a vaginal barbell to your regular Kegel exercise routine can help. They are available at specialty stores that carry sex toys or sexual enhancement items.)